There is an excellent eluceo/ical package that allows you to easily create ics files.
Here is an example usage from docs:
// 1. Create new calendar
$vCalendar = new \Eluceo\iCal\Component\Calendar('www.example.com');
// 2. Create an event
$vEvent = new \Eluceo\iCal\Component\Event();
$vEvent->setDtStart(new \DateTime('2012-12-24'));
$vEvent->setDtEnd(new \DateTime('2012-12-24'));
$vEvent->setNoTime(true);
$vEvent->setSummary('Christmas');
// Adding Timezone (optional)
$vEvent->setUseTimezone(true);
// 3. Add event to calendar
$vCalendar->addComponent($vEvent);
// 4. Set headers
header('Content-Type: text/calendar; charset=utf-8');
header('Content-Disposition: attachment; filename="cal.ics"');
// 5. Output
echo $vCalendar->render();